Optically anisotropic materials are of crucial significance in modern optical applications. The well-known achievements in information display technologies are based on development of anisotropic optical retardation layers made of materials such as liquid crystals (LC) and different types of functional polymers (Broer, D. J.; van Haaren, J. A. M. M.; van de Witte, P.; Bastiaansen, C. Macromol. Symp. 2000, 154, 1). The functional properties of these materials depend on their phase state. Symmetry of a particular bulk phase and its supramolecular structure define the macroscopic performance of a material—its optical, mechanical and other properties. One example is thermotropic liquid crystals. Depending on temperature, thermotropic liquid crystals can be in different thermodynamically stable phase states. However, it is only for the nematic phase that a set of bulk properties (allowed by symmetry) supports their ubiquitous application in optics and information displays. Thermotropic liquid crystal materials of a single type of molecule typically show a very narrow temperature range for the nematic phase (just a few ° C.). The problem was solved by developing liquid crystal mixtures, which in many instances can also be named as ‘guest-host systems’ (Bahadur, B. in Handbook of Liquid Crystals; Demus, D.; Goodby, J., Gray, G. W.; Spiess, H.-W., Vill, V. Eds.; Vol. 2A; Wiley-VCH: Weinheim, 1998, 257). Due to the ‘guest-host’ approach, the thermotropic liquid crystal materials provide the nematic phase in a temperature range larger than 100.C. Guest-host systems are well-known in supramolecular chemistry (Anslyn, E. V.; Dougherty, D. A. Modern Physical Organic Chemistry; John Wiley New York, 2006) and biology (Lodish, H.; Berk, A.; Kaiser, C.; Molecular Cell Biology, 6th ed.; Freeman, 2008). They are composed of two or more molecules or ions held together in unique structural relationships by forces other than those of full covalent bonds (Lodish, H.; Berk, A.; Kaiser, C.; Molecular Cell Biology, 6th ed.; Freeman, 2008). In liquid crystals, the phenomenon of dissolving and aligning of any molecule or a group of molecules such as dyes, impurities or even mesogenic molecules by a liquid crystal can be called a guest-host phenomenon (Bahadur, B. in Handbook of Liquid Crystals; Demus, D.; Goodby, J., Gray, G. W.; Spiess, H.-W., Vill, V. Eds.; Vol. 2A; Wiley-VCH: Weinheim, 1998, 257). The guest molecules couple to the anisotropic intermolecular interaction field of the liquid crystal, but can diffuse rather freely within the host. The essential feature is that a guest-host system is composed of a single material without phase separation. The ability of a material to form the nematic phase is very important for many reasons. For instance, the anisotropy of the viscous-elastic properties of the nematic phase allows fine alignment of the molecules on solid substrates by shear flow (Bobrov, Y.; Kuchenkova, O.; Kouznetsov, M.; Lazarev, P.; Manko, A.; Nazarov, V.; Ovchinnikova, N.; Paukshto, M.; Protsenko, P.; Remizov, S.; J. SID 2004, 12/2, 125). This way the optically anisotropic thin solid retardation layers can be deposited onto different substrates. Unfortunately, thermotropic liquid crystals demonstrate the nematic phase in a specific narrow temperature range. Thermotropic liquid crystals do not allow creating thin solid films with a controlled optical anisotropy.
In a typical LCD, the liquid crystal layer is placed between two polarizers. Off-axis contrast drop is one of the inherent drawbacks arising from the optical anisotropy of LC materials and a feature of the propagation of light through crossed polarizers. For instance, at oblique viewing directions, the light leakage can appear through the crossed polarizers, which increases with increasing off-axis angle. Thus the light leakage caused by both LC optical anisotropy and polarizers should be suppressed. This problem of cumulative off-axis contrast ratio decrease is solved using phase retardation plates comprising retardation layers which are produced from optically anisotropic materials. The appropriate retardation plates provide a small color shift and high off-axis contrast (contrast ratio at wide viewing angles) to the LCD. In the latter case coating is followed by solvent evaporation. Additional alignment procedures are involved such as application of an electric field or using alignment layers produced by rubbing or photo-alignment. This process can be used to manufacture uniaxial optical retardation layers with positive and negative optical anisotropy (Harding, R.; Parri, O.; Marden, S.; Skjonnemand, K.; Verrall, M.; Fiebranz, B. In Proceedings of IDW'06, Otsu, Japan, 2006; Society for Information Display: Q13 Campbell, Calif., 2006; p. 307), biaxial retarders for the successively coated layers (Tilsch, M. K.; Hendrix, K.; Tan, K.; Shemo, D.; Bradley, R.; Erz, R.; Buth, J. Thin Solid Films 2007, 516, 107, Seiberle, H.; Benecke, C.; Bachels, T. SID Symposium Digest 2003, 34, 1162), and optical retardation layers with complex space distribution of the local optical axes (tilted and splayed) for optical compensation of twisted nematic (TN) LCD mode (Oikawa, T.; Yasuda, S.; Takeuchi, K.; Sakai, E.; Mori H. J. SID 2007, 15/2, 133). Photo-aligning and a phase retardation function could be combined into a single material (Seiberle, H.; Bachels; T. Benecke, C.; Ibn-Elhaj, M. In Proceedings of IDW'06, Otsu, Japan, 2006; Society for Information Display: Campbell, Calif., 2006; p. 33). Ultraviolet (UV) curable materials can be used for high-resolution patterned retarders for transflective LCDs (Hasebe, H.; Kuwana, Y.; Nakata, H.; Yamazaki, O.; Takeuchi, K.; Tsai, J.-C. SID Symposium Digest 2008, 39, 1904).
Most of the phase retardation layers used in modern LCD technology is produced by means of mechanical stretching of the extruded or casted polymers. These materials possess the limited opportunity to control an optical anisotropy. Thus control of optical anisotropy is achieved by adjusting stretching parameters as well as by using special additives capable of increasing or suppressing the material anisotropy. A polymer phase retardation layer, for instance, can be attached to a PVA (polyvinyl alcohol) polarizer sandwiched between protective layers. On the other hand, retardation layers can combine both optical compensation and protective functions. For example, polycyclo-olefins are used for manufacturing of phase retardation layers for optical compensation of vertical alignment (VA) and in-plane switching (IPS) LCD modes, while at the same time supplying protective function. However, polycyclo-olefin based phase retardation layers as well as other hydrophobic polymeric materials have a problem of adhesion to the hydrophilic PVA layer. Besides, even in case of hydrophilic stretched phase retardation layers such as triacetylcellulose improving their performance is difficult due to limitations of the mechanical stretching process, especially for the manufacture of large screen displays.
Other drawback of the stretched retardation layers is their small optical anisotropy. Typically, the stretched retardation layers possess small optical anisotropy (Δn=0.001−0.005). The reason is that combined functions of retardation layer and PVA polarizer protection layer are usually reached if the plastic layer possesses mechanical strength achieved at certain thickness of about 50 μm. Such layer should also have high optical quality (e.g. low haze value) and be easily used in a technological process. An alternative to such ‘thick’ retardation layers is a thin coating realized on a plastic or glass substrate and embedded into a conventional polarizer or inside a LC cell, respectively. There is a large group of coatable retarders based on cross-linkable thermotropic liquid crystals. Production of such retardation layers comprises coating of a melt or solution onto a substrate. In the latter case, coating is followed by solvent evaporation. Additional alignment procedures are involved such as application of an electric field or using alignment layers produced by rubbing or photo-alignment. This process can be used to manufacture uniaxial optical layers with positive and negative optical anisotropy, biaxial retarders for the successively coated layers, and retardation layers with complex space distribution of the local optical axes (tilted and splayed) for optical compensation of twisted nematic (TN) LCD mode. Photo-aligning and phase retardation function could be combined into a single material. Ultraviolet (UV) curable materials can be used for high-resolution patterned retarders for transflective LCDs. An alternative, simple and cost-effective method of producing optical films is coating of LLC solutions, where small molecules are capable of self-assembling in columnar supramolecules. These compounds are also known as chromonics and consist of amphiphilic molecules with flat conjugated core and polar solubilising groups at the periphery. The self-assembly in aqueous solution is based on p-p interaction between aromatic cores and on the hydrophobic effect. The rod-like supramolecules in aqueous solution form the nematic type of liquid crystalline state, where the axes of the supramolecules within one domain are aligned along some preferable direction. In the course of deposition of a liquid crystalline solution onto a substrate (coating), an external shear force is applied and all supramolecules become aligned along the shear force direction. Such shear flow alignment is well known in thermotropic LCs and can be explained by anisotropy of the viscous-elastic properties of the nematic phase. Other approaches can be used in order to align LLC, for instance coating of lyotropic liquid crystalline solution onto the photo-aligned layer. Evaporation of solvent fixes the ordered structure, leading to formation of a solid birefringent film with macroscopic optical anisotropy. Molecules in these retardation layers are usually packed with their minimal polarizability axes along the coating direction corresponding to the minimal principal refractive index.
In order to solve the foregoing drawbacks, according to the present invention, there is provided lyotropic liquid crystal (LLC) guest-host systems which allow creating thin solid retardation layers with a controlled optical anisotropy. While thermotropic liquid crystals show the nematic phase in a certain temperature range, the nematic phase of the disclosed lyotropic liquid crystals exists in a certain range of concentrations of materials in solution.